1. Field of the Invention
This invention pertains to field of electronic communication. More particularly, the invention pertains to antenna design. By way of further characterization, this invention pertains to an electronically steerable antenna. By way of further illustration but without limitation thereto, this invention will be described as it pertains to an antenna useful in missile applications.
2. Description of the Prior Art
It is known to use adaptive array antenna arrays to reduce external interference reaching a receiver by shaping the antenna pattern to steer nulls, or minima of sensitivity, in the directions of the interference. This general technique has been in existence for some time. All known adaptive arrays, to date, use weighting on every element followed by an electronic summing network which combines the signal from each of the antenna elements electronically. That is, the signal received by each antenna element is weighted in amplitude and/or phase by means of electronically controlled phase shifters, attenuators, amplifiers, etc. These weighted signals are then summed together to form an array output. This array output is monitored continuously and the weights at each antenna are changed to minimize interference power in the received signal.
Although satisfactory for many purposes, adaptive arrays of the known type encounter problems when the number of elements of the array are spaced closely to each other, less than a one-quarter of a wavelength, due to interaction between the elements. This interaction causes the pattern to vary sensitivity in response to the weight values which in turn limits the depth, width, and the pointing accuracy of the pattern nulls. Such failings have made adaptive array techniques of little value in compact arrangements such as required in the airborne missile arts. Another unfavorable characteristic of the prior art adaptive arrays is the complexity and cost of the weighting circuitry and its correlation loop. Further, this type of construction is complicated by having circuitry for those adaptive arrays that operate at an intermediate frequency. For such arrays an expensive mixing stage is required for each weighted element.